The present invention relates to a power steering apparatus for detecting a steering force to be applied to a steering wheel, and driving an electric motor so as to assist steering while using a hydraulic pump driven by the electric motor as a generating source of a hydraulic pressure when the detected steering force is larger than a predetermined value.
In a power steering apparatus which assists steering by using a hydraulic pump driven by an electric motor as a generating source of a hydraulic pressure, it is considered that when a steering wheel is not operated and a vehicle travels straight, the electric motor is terminated so that an electric power consumption is reduced.
FIG. 1 is a block diagram showing a configuration of such a power steering apparatus. In this power steering apparatus, a motor driving circuit 2 applies a voltage to an electric motor 4 according to a target value of an applied voltage (drive voltage) instructed by a drive voltage determination unit 1c so as to rotate the electric motor 4. The electric motor 4 drives a hydraulic pump 5 so as to discharging a hydraulic oil to a control valve 6 and generate a hydraulic pressure.
When a steering wheel 7 is operated and a gear apparatus (not shown) including a pinion gear 6c provided at a lower end of a steering wheel shaft is actuated, the control valve 6 controls a pressure of a hydraulic oil to be press-fed to pipe lines 6a and 6b connected with a hydraulic cylinder (not shown). As a result, the hydraulic cylinder is actuated, and a steering assisting force according to an operating amount is generated in an operating direction of the steering wheel 7.
The hydraulic oil circulates a hydraulic circuit composed of the hydraulic pump 5, control valve 6, hydraulic cylinder, a supply tank 5a and the like.
A rotational velocity sensor 4b for detecting a rotational velocity of the electric motor 4 is added to the electric motor 4, and a rotational velocity detection signal outputted by the rotational velocity sensor 4b is given to a rotational velocity detection circuit 4a so as to be converted into a rotational velocity signal and be given to a calculation circuit 1b.
The calculation circuit 1b subtracts the rotational velocity signal outputted by the rotational velocity detection circuit 4a from a rotational velocity instruction signal outputted by an instructing rotational velocity determination unit 3 so as to input the resultant into the drive voltage determination unit 1c. A rotational velocity of the electric motor 4 is set in the instructing rotational velocity determination unit 3 so that the hydraulic pump 5 can generate a hydraulic pressure which is required for the steering assistance.
The drive voltage determination unit 1c instructs the motor driving circuit 2 about an applied voltage by feedback control (PI control) based on a difference between the rotational velocity instruction signal outputted by the instructing rotational velocity determination unit 3 and the rotational velocity signal outputted by the rotational velocity detection circuit 4a.
Meanwhile, a steering force detector 7a composed of a torque sensor and the like for detecting a steering force applied to the steering wheel 7 is provided to the steering wheel shaft of the steering wheel 7, and a steering force signal outputted by the steering force detector 7a is given to a steering force detection circuit 7b. When the given steering force signal indicates a steering force which is larger than a predetermined steering force, the steering force detection circuit 7b turns on a power supply of the motor driving circuit 2. The motor driving circuit 2 whose power supply was turned on starts the electric motor 4 by the applied voltage instructed by the drive voltage determination unit 1c thereat.
In addition, on the contrary, when the given steering force signal indicates a steering force which is smaller than the predetermined steering force, the steering force detection circuit 7b turns off the power supply of the motor driving circuit 2.
Here, the similar technology to the above-mentioned power steering apparatus is disclosed in Japanese Patent Application Laid-Open No. 58-156457 (1983).
In the above-mentioned power steering apparatus, when the motor driving circuit 2 is turned on and the electric motor 4 is started by the applied voltage instructed from the drive voltage determination unit 1c thereat, as shown in FIG. 2, the rotational velocity of the electric motor 4 overshoots, and accordingly an unnecessary large inrush current is generated and unnecessary electric energy is consumed. Moreover, in some cases, the rotational velocity is not stabilized immediately.